1. Field of the Invention
The present invention relates to a receiver for receiving a digital modulated signal such as a QPSK signal.
2. Description of the Related Art
In digital broadcast making use of a communications satellite, video and audio are broadcasted by having the carrier subjected to digital modulation. In the communications satellite (CS) audio broadcast, digital modulation having a data rate of 24.576 Mbps is used. In video-audio broadcast practiced in Japan, digital modulation having a data rate of 42.192 Mbps is used.
The radio wave from the satellite is a frequency band of 11.2 GHz. The radio wave received by an antenna is converted by a converter into a first intermediate frequency of a frequency band of 1-2 GHz. Conventionally, a dielectric resonator is used as the local oscillator in the converter. Since the dielectric resonator lacks in stability, the frequency given by the local oscillator sometimes exhibits a drift of around xc2x12 MHz. Therefore, to tune to the first intermediate frequency signal from the antenna, the voltage-controlled oscillator (VCO) control loop requires a synchronization locking control means for a frequency range of at least exceeding xc2x12 MHz or, preferably, about xc2x15 MHz.
A conventional art example for solving such a problem is disclosed in Japanese Laid-open Patent No. S 62-136152.
FIG. 5 is a block diagram showing a conventional receiver for a QPSK modulated signal.
The conventional art example will be described with reference to FIG. 5.
Mixer 102 mixes a signal of a frequency band of 11.2 GHz received by antenna 101 with an output signal of local oscillator 103 to output a first intermediate frequency signal of a frequency band of 1 GHz. Multiplier 102 and local oscillator 103 constitute a converter attached to the antenna.
Mixer 104 mixes the first intermediate frequency signal with an output signal of local oscillator 105 for channel selection and thereby outputs a second intermediate frequency signal of a frequency band of 400 MHz.
The second intermediate frequency signal limited for bandwidth by bandpass filter (BPF) 106 is supplied to Quardrature Phase Shift Keying (QPSK) demodulator 200.
QPSK demodulator 200 is made up of detectors 201 and 202, phase shifter 203, waveform shapers 204 and 205, multipliers 206 and 207, subtractor 208, adder 209, low-pass filter (LPF) 210 and voltage-controlled oscillator (VCO) 211.
Phase shifter 203 shifts 90 degrees the phase of the output signal of VCO 211.
Adder 209 adds the output signal of low-frequency oscillator 110, passed through switch 111, to the output signal of subtractor 208.
Switch 111 is closed when demodulator 200 is in its asynchronous state, while it is opened when demodulator 200 is in its synchronous state.
When demodulator 200 is brought into its synchronous state, a detection signal is delivered from synchronous pattern detector (SPD) 109 in digital signal processor (DSP) 108. Switch 111 is controlled by this detection signal.
The capture range of QPSK demodulator 200 is around xc2x1500 kHz, while the lock range thereof is xc2x1several MHz.
When local oscillator 103 is drifting several MHz, QPSK demodulator 200 sometimes becomes unable to perform synchronization locking. In such case, switch 111 is closed and a low-frequency signal is added to a phase error signal, so that VCO 211 is swept and, thereby, synchronization locking control is performed.
This makes use of the characteristic of the VCO control loop that it has a wide lock range while it has a narrow capture range, and, hence, the sweeping expands the apparent capture range of the loop. By means of such a configuration, even when the carrier frequency is off-centered, it can be captured and demodulated.
Recently, as applications making use of communications satellites, uses of broadcast communications, such as inter-company communications, have become increased. In such communications applications, not so large an amount of transmitted data is required as in ordinary video broadcasting. Therefore, the data rate is 8 Mbps, 4 Mbps, or so, which is considerably lower than the data rate around 40 Mbps in ordinary broadcasting.
Since the carrier bandwidth is in direct proportion to the data rate, the space between adjacent carriers can be set narrower according as the data rate becomes lower. Hence, efficiency in utilization of the frequency can be improved.
When the data rate is 8 Mbps, the distance between channels in the carrier disposition is about 5.4 MHz, while it is about 2.7 MHz when the data rate is 4 Mbps.
If synchronization locking control over a frequency range of xc2x15 MHz, in accordance with the data rate for conventional broadcasting, is applied to a carrier at a low data rate of 8 Mbps or 4 Mbps, there arises a problem that a wrong channel comes to be selected.
The present invention was made to solve the above mentioned problem encountered in a receiver arranged to support multiple data rates. In order to solve the problem, the receiver of the present invention comprises:
tuning means for converting a desired modulated carrier wave selected from a first intermediate frequency signal received by an antenna into a second intermediate frequency signal;
a bandpass filter for limiting the second intermediate frequency signal for bandwidth;
orthogonal detecting means supplied with an output signal of the bandpass filter for delivering an I baseband signal and a Q baseband signal orthogonal with each other;
a voltage-controlled oscillator for feeding a signal for detection to the orthogonal detecting means;
AD converters for converting the I and Q baseband signals into digital signals;
digital demodulator means supplied with the I and Q baseband signals digitized by the AD converters for delivering a frequency error signal, a synchronism detection signal, and a demodulated signal;
sweeping control means supplied with the frequency error signal and sweeping data for delivering a control voltage of the voltage-controlled oscillator; and
a microcomputer supplied with a channel select command including carrier wave information and data rate information of the first intermediate frequency, the frequency error signal, and the synchronism detection signal, in which the microcomputer delivers a tuning frequency setting value to the tuning means, and delivers the sweeping data to the sweeping control means.
By virtue of the structure as described above, the sweeping frequency range of a desired carrier is calculated on the basis of the data rate of the carrier of the selected channel and sweep control is executed within the thus calculated sweeping frequency range so that synchronization locking to the carrier is attained and, thereby, an erroneous channel selection can be prevented.